Secondary literature means "books about books" - in the context of English studies, the term refers to works of criticism, either about a single author or a group of authors, and to surveys of a particular period or genre. Not all secondary literature is equally valuable or authoritative, and one of the expectations of university-level work is that you evaluate your sources critically. This means, among other things, only including direct quotations when they are contextualized by your framing comments.
Making properly critical use of secondary literature involves judging just how relevant a particular source is to the argument you're making - a piece of criticism published fifty years ago is unlikely to correspond to the current state of the academic field in Romantic studies, for example, but might be a valuable piece of evidence if you were writing about the cultural climate of the 1950s. Review articles, which offer a review of the literature in a field, can be a very helpful source of guidance about the significance and relevance of particular items of secondary literature.
One essential question to ask when evaluating the worth of a piece of secondary literature is whether it has been through a peer review process, where its argument is examined by other people - this is the major difference between print-based publication, and most forms of web-based publication